When a family moves into a San Francisco apartment, an opportunistic troll decides to make his move and take possession of little Wendy (Jenny Beck), thereby paving the way for new troll recruits, the first in his army that will take eventual control of the planet.
As luck would have it, the building conveniently rests on a crease in the time-space continuum, so it isn't hard to open the door to trollworld, or whatever the troll would call it.
But even the path to world domination begins with a single step, so the troll begins by disposing of each tenant with his pointy magical ring; each victim then turns into a fern (this may be the only existing film in which the late U.S. Representative Sonny Bono becomes a houseplant).
Fortunately for the human race, the next door neighbor is a kindly old witch who has just the game plan to win back Wendy (now under the troll's influence as evidenced by her poor social skills and equally bad eating habits), vanquish the paranormal pest, and save the human race from imminent troll tyranny.
The stage is then set for inevitable battle between the good witch and the evil troll for control of the world, but first she and the brave family must combat a twenty-foot tall ubertroll who looks akin to a Furby doll left in the sandbox over a long, hard Minnesota winter.